An elementary illustrated introduction to simplicial sets
Greg Friedman, 2008.
A leisurely introduction to simplicial sets
Emily Riehl, 2008.
A Primer on Homotopy Colimits
Dan Dugger, 2008.
Simplicial objects in algebraic topology
Peter May, 1967.
Simplicial Homotopy Theory
Goerss and Jardine, 1999.